Instrumentation

The I&C system also includes the erection of panels and the laying of cables as is done for the electrical system. The additional features in the I&C system for the purpose of construction are installation of various types of tubing systems, and the advanced termination of wires. The I&C system involves a large amount of process parameter sensing equipment, field equipment, analysers, data recorders and their associated wiring and tubing.

The network of wiring and tubing of the I&C system is well spread all over the plant. Major emphasis is therefore placed on the layout of various types of tubing systems for the I&C, and proper termination of the wiring system.

Designers do not usually provide the field layout drawings for the tubing system, as this layout has to be decided to suit the field conditions. This situation may not be preferred, since the quantity of the material required may be difficult to estimate and hence remain undefined for some time. Advancements in tubing layout define a corridor system for laying of the tubing, which is earmarked at the time of initial design. This corridor system developed during the 3-D model not only improves the aesthetics, but is also operation friendly. Fairly accurate estimation of quantities is also ensured.

The use of automatic tube welding machines capable of welding of tubes with different sizes and wall thickness varying from 6–25 mm has reduced erection time. Automatic tube-to-tube welding machines are available on the market. In order to achieve quality welds, all parameters such as tube thickness, tube size, speed of rotating head and type of arc required to be generated, are pre-decided. The advancement in these machines has further increased the output capacity as compared to the machines used in the past.

Three dimensional modelling of tubing layout for a congested system is first developed, using any of the plant design management tools. This is especially meant for core monitoring systems, and process control of the main nuclear supply system and reactor regulation and protection system. A proper corridor is framed in order to avoid any group interference or damage during installation. Use of perforated light channels for laying this delicate tubing as an advanced construction methodology ensures a well-developed layout with proper identification. Each tube is identified by a predefined number that is stamped on it by a stamping machine, using halide-free ink. Figure 64 shows prefabricated tubes.

A well established I&C calibration laboratory has to be set-up before the I&C activity takes place. Calibration of all I&C instruments is considered essential to ensure that commissioning is completed in the scheduled time, and that defective or damaged instruments are segregated before their installation.

FIG. 63. Prefabricated panels.

Supports for instruments and tubes

Vertical channel frames, H-type or different shapes or size of frames have conventionally been used to suit the field measurement for supporting sensing equipment, transmitters and various I&C instruments, etc. The fabrication of supports is mostly related to ‘as-built’ field measurements and location of process instruments, which takes time and causes congestion in the working space, since all instrumentation jobs are conventionally carried out on completion of major field activities. As a general practice, most of the support pedestals are fabricated in situ in the field. This leads to the development some sort of fabrication shop in the plant area, thus interfering with other plant workers who are fully involved in commissioning activities during such a critical period. This situation is also not advantageous from an industrial safety point of view.

Prefabricated perforated channel type frames are now being developed to handle all types of installation arrangements during laying of tubing for I&C systems. Figures 65 and 66 show the typical support arrangement and height, respectively.

The height of these supports can be suitably adjusted using couplers and extension pieces, thus considerably saving on installation time.

FIG. 64. Prefabricated tubes.

FIG. 65. Typical frame support arrangement.

Root valves and fittings

Root valves and tube fittings are also an integral part of I&C systems. As root valves provide isolation to the entire measurement system, their location during installation is extremely important. Isolating root valves are provided at take-off connections to isolate the entire measurement system from the main pipeline/vessel when necessary.

The recommended practices which should be followed are:

— Root valves should not affect the pressure signal during normal operation;

— Root valves should be capable of withstanding the maximum working pressure and temperature of the piping/vessel system to which the take-off adapters or nipples are attached;

— Isolating/root valves should be located immediately following the tapping point;

— It is preferable to use positive isolation valves for root valves;

— Root valve bore should not be less than the inside diameter of impulse tubing/piping;

— The root valve may or may not perform the function of the accessible isolation valve, depending on its location;

— Root valves and the pipes to which they are connected should be of the same material.

Tube fittings

Tube to tube fittings are being extensively used as a matter of convention in various type of tubing for I&C.

This includes tube to tube connector elbows, reducers and bore-through fittings. Tube fittings are used to join or connect a tube end to another member. Any tube fitting must accomplish two important functions within the pressure, temperature and vibration criteria that the tube fitting is designed to meet. Firstly, the tube fitting must grip the tube end so as to prevent loss of seal or tube blowout. Secondly, the tube fitting must maintain a primary seal against leakage.

FIG. 66. Frame support height.

It is recommended that tube fittings meet the following requirements:

— Offer reliable installation over a range of field conditions, since improper make-up and tightening remain the leading causes of leakage;

— Cope with the wide variation in tubing characteristics, including differences in wall thickness, hardness, ovality and burst pressures;

— Be of a compatible material with the tubing or pipe material on which they are used to avoid electrolysis and to provide acceptable weld joints;

— Be used at pressure temperature ratings not exceeding the recommendation of the tube fitting manufacturer, and meeting the environmental and process system requirements;

— Be installed in accordance with manufacturer’s recommendations;

— Withstand the temperature and pressure cycling as per the appropriate standard;

— Provide a sufficiently robust grip on the tube such that when a tensile load (e.g. during hydro test or during operating conditions) is applied to it, the tube does not pull out of the grip. Generally, the acceptable pull-out tension load is more than four times the hydrostatic test pressure load.

The following types of fittings are generally used:

— Flareless compression type (single terrule);

— Flareless compression type (double terrule);

— Bite type;

— Flared fitting.

Though flared fitting and bite type fittings are more economical than double ferrule flareless fittings, double ferrule compression fittings have an edge over other type of fittings. They have an enhanced sealing action against any leakage or vibration; fatigue resistance; a greater margin of performance; and are easy to install. These types of fittings are available in various materials as per the code requirements, and their use enhances productivity.

4.3.2.10. Control room

Over a period of time, most of the advancements have taken place in developing the new concept of control panel to provide a user-friendly human–machine interface. The use of modular types of control panels is preferred over the conventionally made control panels that are wired after installation.